Why everyone is using CNN for image segmentation?
CNN is used since it is effectively an optimized use case for dealing with image data. 

CNN effectively automatically extracts features from images. Other techniques are more likely to not take full advantage of the data. CNN is able to make full use of the data by also including information from adjacent pixels and downsample through layers.  


Here is a paper on the performance of CNN on image data
Here is a paper comparing CNN's to other methods